Confusingly, not only are the same dimensions of battery are used for Silver Oxide and Mercury cells, but often the same names are used as well! Even though the chemistries and the voltages produced are different. It is wise to take care examining the expired battery or, better still, checking with the manufacturers documentation to check that an Alkaline battery of 1.5V is required.

If you find you require a 1.35V mercury cell, you are in difficulty. Mercury cells are no longer manufactured because of their toxicity and environmental unfriendliness. Look at the PX625 page for advice.

The LR9 is the least expensive PX625 replacement. They fit perfectly but, as with all alkaline cells the voltage is not stable, it drifts down from 1.55V towards 1.35V (the voltage of the original Mercury cell) and then lower still. If you use the cell in a camera that needs a steady reference voltage, as the voltage is not steady throughout the useful life of the cells, you might need to constantly adjust how you compensate.

An alternative mercury free replacement for the PX625 is the silver oxide S625PX. These S625PX cells produce 1.55V which is higher than the 1.35V produced by the original PX625 mercury cell. Because the S625PX silver oxide cell produces a steady 1.55V, you are able to compensate for the higher voltage by a fixed number of stops or by adjusting the ASA setting. There may be as many as two f-stops underexposure by a camera that uses 1.55v instead of the original 1.35v, but the compensation you require is constant.